Malzahar can only cancel his ult by reativating it.
I don't know if its just me that has this problem, but I screw up his ult every time.
I got in for the combo, and accidentally try to use an ability after I ult, forcing it to cancel. It happens a lot in the heat of the moment because I am trying to use everything.
I just think it would be a little easier for him to just reactivate it to cancel it early.